For a quality test of a butt-welded portion of a small-bore tube, such as a boiler tube, a radiation transmission test and an ultrasonic flaw-detection test are currently performed. In ultrasonic flaw detection, various flaw detection methods are used, such as an angle beam testing method for flaw detection using transverse ultrasonic waves and a creeping flaw-detection method using creeping waves (for example, see Patent Literatures 1 and 2).
In the radiation transmission test, another operation, such as a welding operation, cannot be performed at the same time because outsiders need to be evacuated from a controlled area during the test. Furthermore, because of a problem of workers' exposure to radiation, it is desirable to change the radiation transmission test to the ultrasonic flaw-detection test, if possible.
On the other hand, in the ultrasonic flaw-detection test, it is known that the above-described angle beam testing method for flaw detection has an advantage that, by making ultrasonic waves skip on the inner surface of the tube, the presence or absence of a flaw in the entire plate thickness of the tube can be detected with higher precision than in the creeping flaw-detection method.
On the other hand, in the creeping flaw-detection method, there is a known advantage that, since ultrasonic waves spread, the presence or absence of a flaw in the entire plate thickness of the tube can be detected in a shorter time than in the angle beam testing method for flaw detection.
Therefore, in some cases, a test for the presence or absence of a flaw is performed by using one or both of the angle beam testing method for flaw detection and the creeping flaw-detection method, depending on the object to be subjected to the ultrasonic flaw detection.